1. Field of the Invention.
This invention generally relates to power amplifier (PAs), and more specifically, to detecting and compensating for saturation of power in PAs.
2. Related Art.
In GSM-compliant or TDMA-compliant mobile devices, such as wireless handsets, the transmitter needs to power up and down within a predetermined power vs. time mask having predetermined noise characteristics. At the same time, in order to increase battery life and hence talk time, the device should be operated at maximum or near-maximum efficiency. These objectives are difficult to achieve if the power amplifier in the transmitter or transceiver becomes saturated.
Conventional techniques for saturation compensation are open loop techniques in which, upon the detection of a saturation condition, the amplifier is backed off by a predetermined amount. Depending on the degree to which the amplifier has entered the saturation mode, the predetermined amount may be insufficient to bring the amplifier out of saturation, or may back the amplifier off by an amount which is greater than that necessary to exit the saturation mode. Thus, with the conventional techniques, there is a risk that the amplifier will not be operated at maximum or near-maximum efficiency.
This invention provides a system for detecting and compensating for a saturation condition in a power amplifier. The system may include in series a saturation compensation circuit, a comparator, an integrator, and a saturation detection circuit. The saturation compensation circuit receives a control signal for controlling the shape of the output of the power amplifier, and adjusts that signal when a saturation condition is detected by the saturation detection circuit. The comparator compares the output of the saturation compensation circuit with the output of the power amplifier and produces an error signal representative of the difference between the two. The integrator integrates the error signal to produce a cumulative error signal. The saturation detection circuit detects a saturation condition responsive to the integrated error signal, and signals the saturation compensation circuit when a saturation condition has been detected. Responsive to the detection of a saturation condition, the saturation compensation circuit adjusts the control signal by a compensation value that is derived from the value of the error signal at the time the saturation condition is detected. In one implementation, the compensation value may be equal to the instantaneous error at the time saturation is detected multiplied by a predetermined constant. In another implementation, this compensation value may be the average error at the time saturation is detected multiplied by a predetermined constant. A signal derived from the output of the adjusted control signal forms the input to the power amplifier. In one implementation, the input to the power amplifier is derived from the output of the integrator.
In another implementation, a saturation condition may be detected when the cumulative error output from the integrator equals or exceeds a predetermined value, INTEG_MAX, determined during device calibration. In response, the saturation detection circuit sets a flag to 1. When the value is set to 1, the saturation compensation circuit latches a compensation value equal to the instantaneous error output from the comparator by a predetermined calibration constant. The saturation compensation circuit subtracts this value from the signal representing the desired output of the power amplifier.
A method for detecting a saturation condition of a power amplifier is also disclosed. A signal derived from or representative of a control signal for controlling the shape of the output of the power amplifier is compared to a signal representative of or derived from the actual output of the power amplifier, and the two are differenced to form an error signal. The error signal is then integrated to form a cumulative error signal. The saturation condition is detected if the cumulative error signal equals or exceeds a predetermined value.
Another method of the invention includes compensating for the saturation condition of a power amplifier. When a saturation condition is detected, a signal representative of or derived from a control signal for controlling the shape of the output of the power amplifier is compared to a signal representative of or derived from the actual output of the power amplifier, and the two are subtracted to form an error signal. A compensation value is derived from the error signal and is subtracted from the control signal. The input to the power amplifier is derived from this adjusted signal.
The compensation value may be determined by multiplying the instantaneous error, at the time saturation is detected, by a predetermined constant. In another implementation, the compensation value may be determined by multiplying the average error signal, at the time saturation is detected, by a predetermined constant.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.